The human skeleton has a number of functions: it protects internal organs, provides muscle attachment sites to facilitate limb movement, acts as a store for calcium and phosphorus, and in some instances produces red blood cells[1]. The adult skeleton contains some 206 bones[2] any one of these may at some time in an individuals life require reconstruction or replacement.
Tissue engineering is defined as the application of principles and methods of engineering and life sciences toward fundamental understanding and development of biological substitutes to restore, maintain and improve tissue functions. Tissue engineering can be applied to both hard and soft tissues. Hard tissue includes, for instance bone and teeth, whereas soft tissue includes, for instance, organs, blood vessels, muscles, ligaments, tendons, cartilage, and nerves.
Implants used for tissue engineering ideally have the following characteristics;                They are biocompatible; that is, they can function in the body without creating either a local or a systemic rejection response.        They have mechanical properties that duplicate the structures they are intended to replace; for example, they are strong enough to take weightbearing loads, flexible enough to bear stress without breaking and able to move smoothly against each other as required.        
Bone is highly vascular i.e. it contains a network of blood vessels that support rapid healing when required. A problem arises when defects reach a certain size at which point the bone is unable to heal itself. This is known as a critical sized defect, the size at which a defect becomes critical will depend on the age and health of the individual. Such defects may arise as a result of trauma, disease, genetic abnormality, infection, tumour growth or degenerative disease (for example osteoarthritis).
In relation to the manufacture of a bone implant there is also a serious disconnect between the desire for porosity and strength, since these physical characteristics are mutually exclusive. Bone is a biological composite, it consists of collagen (a fibrous protein) and crystals of hydroxyapatite (HA), a ceramic. The collagen confers flexibility and fracture toughness to the matrix, whilst hydroxyapatite confers stiffness. There are two different types of bone in the skeleton: cortical bone and trabecular bone as illustrated in the femur shown in FIG. 1. These two types of bone which, whilst comprising the same types of cells and material, differ in their structure and how much of the tissue is calcified. Overall cortical bone makes up about 80% of the skeleton, and trabecular bone about 20%, but the proportions of these types of bone differ between different regions of the skeleton.
Cortical bone (also referred to as Haversian bone or compact bone) is a dense calcified tissue (80-90% of the volume is calcified) that forms the outer surfaces of most bones and the shafts of the long bones. It has a low surface area to mass ratio and gives strength and structure to the skeleton and has a Young's modulus of approximately 20 Gpa.
Cancellous bone (also referred to as spongy or trabecular bone) is made up of a network of trabeculae (strands of bone), giving it a characteristic spongy appearance. It has a high surface area to mass ratio and is found mainly at the ends of long bones and in the spine and hip. Only 15-25% of the volume is calcified, the rest of the space is occupied by blood vessels, connective tissue and bone marrow and trabecular has a Young's modulus of only 2 GPa. The main function of trabecular bone is metabolism and is where most bone turnover occurs.
Cortical bone always surrounds cancellous bone although the amount of each varies according to anatomical location. A preferred bone implant replicates the removed bone section in terms of its cortical and trabecular bone geometry.
A range of existing techniques for bone augmentation are currently in use and the basis of these techniques and their inherent advantages and disadvantages are summarized below and in Table 1.
Autografting is the current ‘gold standard’. Autografting is the use of bone from one part of a patient's body for use somewhere else. Bone is typically removed from the iliac crest and then used as required [4].
Allografting is a method that uses bone donated from another human. Typically organ donor's bones are removed post mortem and stored in a bone bank ready for use when a surgeon requires it.
Xenografting is a method in which tissue is harvested from another species and therefore all cells and proteins are removed during processing to prevent an immune response.
Bone Cement can be mixed in the operating theatre and moulded to fit the defect in-situ. After mixing the cement hardens rapidly. Historically PMMA (polymethylmethacrylate) cement was used. This is a two part cement that has been used extensively for hip replacement operations over the past 50 years. However, calcium phosphate and calcium sulphate cements are now being used which are more biocompatible and some are biodegradable.
Biocompatible implants are implants fabricated from materials that have minimal effect on the patient. They may be made of plastic or metal fabricated in-situ by the surgeon or made prior to the operation with rapid prototyping techniques [5].
Coralline is coral harvested from reefs and treated to remove any pathogens. Its structure and make up are very similar to native bone such that it promotes in-growth of bone after implantation and is gradually degraded by the body to be replaced by natural bone [6].
Synthetic bone grafts may be made of ceramics, polymers, or a composite of both. Their properties may vary in terms of mechanical strength, porosity, degradation time, and form.
TABLE 1A summary of the available methods for repairing critical sized bone defects:Graft MaterialADVANTAGESDISADVANTAGESAutograftContains the patient's own cellsLimited amount availableOsteoinductiveDonor site morbidityCortical bone available forIncreased operative time due tostrengthsecond procedure and hence highResorbable.cost.AllograftOff the shelf availability(boneProcessing removes live cellsbank)Possibility of infectionExcellent osteoconductiveImplant quality is variablepropertiesIrradiation can effect propertiesIdeal mechanical propertiesLimited availability and high cost.ResorbableBone CementEasily mouldable to fit.High temperature during cure kills(PMMA)Readily available.cellsCan be injected minimallyUnmixed monomer kills cellsinvasively.Non resorbable.Bone CementEasily mouldable to fit.Limited porosity(Calcium based)Readily available.Limited strengthCan be injected minimallyinvasively. ResorbableBiocompatibleIdeal fitAllergic reactions to materialimplantsBiocompatibleHigh cost of preparing implant(e.g plastic orHigh strengthNon resorbablemetal)CorallineExcellent osteoconductiveHigh CostSynthetic graftpropertiesPoor mechanical properties(e.g ceramics,ResorbableLimited mechanical propertiespolymers, or aOff the shelf availabilityModerate cost.composite ofA multitude of forms availableIncomplete resorbtion.both)ResorbableProperties vary in terms ofNo risk of infectionmechanical strength, porosity,degradation time, form
An ideal synthetic bone implant would incorporate a number of the advantages of the known techniques without any of the associated disadvantages.
There are two main processes used within the field of bone implant manufacture, referred to as (i) rapid prototyping (RP) or (ii) conventional methods which cover those other than RP (e.g. injection molding).
RP refers to a broad category of techniques that can automatically construct physical prototypes from computer-aided design (CAD) models. Rapid prototyping provides the unique opportunity to quickly create functional prototypes of highly complex designs in an additive fashion. This layered manufacturing is also known as solid freeform fabrication (SFF), desktop manufacturing, and computer-aided manufacturing (CAM). There are several methods of RP, but universal to all of them is the basic approach they use, which can be described in three phases; (i) a geometric model is constructed on a CAD/CAM system, (ii) the CAD model is converted into a stereolithograph (STL) and (iii) a computer program reads the generated STL file and slices the model into a finite set of layered cross-sections, each layer then being created individually from liquid, powder, or solid material and stacked onto the previous layer with each layer joined to its neighboring surfaces
Fused deposition of ceramic (FDC) is an example of an RP technique in which a thin strand of material is deposited from a needle onto a moving work piece in order to produce a structure as shown in FIG. 2. Hydroxyapatite (HA) scaffolds can be produced using this method by first preparing a paste of HA, propan-2-ol, polyethylene glycol, and polyvinyl butyral in different ratios and drying to the desired consistency[3] The polymer is completely dissolved in solvent before the addition of ceramic powder to the solution. The mixture is then allowed to dry in an oven at 60° C. for approximately one hour with stirring every 10 minutes with the resulting paste being loaded into a syringe and extruded through a polished hypodermic needle onto the moving work piece. Once the scaffold has been completed it is dried in air at 300° C. followed by sintering at 1250° C.
A particular disadvantage of this technique is that the pastes extruded from milled powder could not be extruded easily and produced irregular filaments that curl on exit from the nozzle. Whilst this problem can be overcome by using ultrasonic dispersion, the mix still requires constant stirring.
Whilst RP techniques enable the creation of complex patient specific geometries in a range of different materials, these techniques suffer from many drawbacks, such as: the machines themselves are expensive; the machines are slow to produce scaffolds; the scaffolds require significant post-processing. Above all RP techniques lack scalability. By this it is meant that they cannot produce high volumes of parts at low cost.
All modern car exhausts include a catalytic convertor. This reduces harmful emissions of hydrocarbons, carbon monoxide and nitrogen oxides into the atmosphere. The catalytic convertor works by converting gases into water vapour and less harmful gases.
An automotive catalyst comprises a high specific surface area substrate, typically ceramic or metal, onto which an active (catalytic) wash-coat is added. The wash-coat is designed to further increase the specific surface area and make as much of the active material as possible available to the exhaust gas to increase the reaction rate. The core of the catalytic converter is typicallly a honeycomb cellular monolithic ceramic substrate with pores that measure less than 1/1000 metre. The pores are coated with a washcoat that contain metals such as platinum, palladium and rhodium.
The preparation of ceramic monoliths from a mixture of cordierite powder and agglomeration agent (e.g. polyethylene oxide or cellulose) for catalyst applications is a well established process.
Surprisingly it has been found that monolithic structures for use as implants in tissue engineering can be manufactured using a similar extrusion process as for the ceramic automotive catalysts.
An object of the invention is to provide an implant for use in tissue engineering which replicates the structure and properties of the natural tissue that it replaces.
An object of the invention is to provide an implant comprising functionally gradient materials.
An object of the invention is to improve the manufacturing process such that the implants can be mass produced in a repeatable, controlled and rapid manner.